The present invention relates to an output stabilizing device and more particularly to an output stabilizing device for use with semiconductor laser oscillators.
The semiconductor laser oscillators have been used in a variety of apparatuses because of their useful features such as small size and light weight. A typical example of the applications to which the semiconductor laser oscillator is applied, is an optical disk apparatus in which an optical disk is irradiated with a laser beam produced from a semiconductor laser oscillator assembled into an optical head for recording information in a high density and playing back the recorded information. The optical disk apparatus can record a great amount of information. Because of this feature, it has application in a document filing system for recording and playing back image information. The semiconductor laser oscillator assembled into the optical head in the optical disk apparatus is sensitive to changes in the ambient temperature and its change is great. That is problematic from an oscillation stability standpoint. An oscillation threshold current of the general semiconductor laser oscillator depends on a junction temperature Tj and is givenby the following expression: EQU Ith exp(Tj/To)
Therefore, when a junction temperature changes by 50.degree. C., the oscillating output power changes in a great range from the power at no oscillation to the maximum oscillation power, even if the drive current is constant. A general countermeasure taken for this problem is to apply feedback to a drive circuit of the semiconductor laser oscillator. A prior feedback method is such that part of the output power of the laser oscillator is detected in the form of a monitor beam and the monitor beam is simply fed back. This method is effective for stabilizing the laser output power when it is applied in the case where the optical disk apparatus is used only for playback, and the semiconductor laser oscillator plays back the playback laser. When applied to an optical disk apparatus for both recording and playing back purposes, however, it fails to obtain stable oscillating output power of both the playback laser and the record laser, because the playback laser output (low output power) range and a record laser output (high output power) range are selectively switched according to the operation mode of a record mode or a playback mode.
In an optical disk apparatus capable of performing both record and playback, the semiconductor oscillator produces the playback laser at low levels at all times. With the playback laser beam, a prepit array or a pregroove formed on the optical disk is traced to effect the tracking. When a modulating signal corresponding to data for recording, which is a pulse signal with a pulse width of several hundreds ns or less, is applied to the semiconductor laser oscillator, the semiconductor laser oscillator is changed to a high level range during the period of the modulating signal supplied to the semiconductor laser oscillator, so that it produces a high level laser beam, i.e. a recording laser beam. At this time, the recording laser beam has energy necessary for melting and deforming a metal film of the optical disk, which is several tens times the playback laser beam, for example. Accordingly, at this time, the monitor laser beam is several tens times or more than that at the time of playback. When such a high level monitor laser beam is detected by the monitor laser beam detector, the detector generates high level monitor pulses to greatly influence the operation of the optical head circuit. The result is that the operation of the optical head is remarkably unstable, possibly causing an erroneous operation.